The present invention relates to an apparatus and method for coating pipes and particularly, but not exclusively, girth welds on oil and gas pipelines.
Oil and gas pipelines are usually formed from many lengths of externally coated steel pipe welded together. The pipes are coated to prevent corrosion. They are usually coated at a factory remote from the site where they are to be laid. This is generally more cost effective than coating them on site. At the factory a coating is applied to the outside of the pipes leaving a short length uncoated at each end. This is necessary to enable the pipes to be welded together end-to-end on site to form a pipeline. Each resulting girth weld and adjacent uncoated region of the pipe must be coated before the pipeline is laid. This coating is known as the field joint coating. Ideally, the properties of the field joint coating should match those of the factory applied coating applied to the remainder of the pipes.
In one known arrangement the factory applied coating comprises polypropylene and the field joint coating is formed by polypropylene tape wound helically over the field joint area and factory applied coating adjacent the field joint area. Each turn of the tape overlaps either part of the factory applied coating or a previous layer or layers of tape by a predetermined amount.
A known method of applying a polypropylene tape field joint includes the following steps. The surface of the factory applied coating which will be overlapped by tape is cleaned and masked. The field joint area is then abrasive blast cleaned and induction heated. A layer of fusion bonded epoxy powder is applied to the field joint area, followed by a layer of polypropylene powder. The heat in the pipe causes the epoxy powder to bond to the pipe, the polypropylene subsequently bonds with the epoxy as it melts and flows to cover the surface of the field joint. The masking on the adjacent factory applied coating is now removed. The surface of the factory applied coating will now have been heated by conduction of heat through the coating from the pipe, but not so much as it melts. A layer of molten polypropylene is then applied to the surface of the factory coating to be overlapped by tape using a flame spraying process.
Polypropylene tape is now wrapped helically across the field joint area and adjacent factory applied coating. Residual heat in the pipe and factory applied coating from induction heating and subsequent flame spraying causes the surface of the polypropylene tape brought into contact with the pipe or factory applied coating to melt so that the tape becomes welded to the pipe or coating on cooling. Bonding of overlapping layers of tape together occurs through conduction of residual heat in the pipe through an underlying layer of tape to an overlying layer of tape sufficient to cause the abutting surfaces of the tape layers to melt. Bonding of layers of tape may also be facilitated by the subsequent external application of heat to the tape using a gas flame.
A problem with this existing method is that heating of the factory applied coating and molten polypropylene bonded thereto during the flame spraying is difficult to control. The margin between insufficient heating, leading to poor bonding between factory applied coating and tape, and overheating, causing burning of the factory applied coating or applied molten polypropylene which also adversely affects bonding with tape, is very narrow. In practice it is difficult to achieve consistently good bonding.
Another problem arises because the polypropylene applied to the factory coating remains molten for a limited period only. If there is too great a delay before application of tape the polypropylene can begin to solidify making it unlikely that tape will bond properly to it. This can happen, for example, where two turns of tape are required to coat the factory applied coating and the polypropylene solidifies between application of the two turns, and where the factory coating adjacent one side of a field joint is coated with tape before that at the opposite side of the joint.
Another problem is poor bonding between layers of tape. Bonding occurs through conduction of residual heat from the pipe and/or application of heat externally using a gas flame. Application of external heat is, in practice, largely ineffective in heating up a thickness of tape and it can take a considerable amount of time for sufficient heat to cause bonding to be conducted through layers of tape from the pipe itself. As the field joint coating increases in thickness there becomes a point where sufficient heat to cause satisfactory bonding cannot be conducted from the pipe. Further, because it is often necessary to force quench a field joint to solidify the polypropylene coating in order for it to withstand mechanical loading, for example from pipe rollers, and in many instances offshore a finished field joint is immersed in the sea within two or three minutes of application, regardless of its thickness a joint is often cooled before bonding between tape layers is complete. Also, the application of external heat using a gas flame despite being largely ineffective is also imprecise and can easily result in burning or complete melting of the outside of the coating, both of which are undesirable. As interlayer bonding and adhesion to the factory coating cannot be verified other than by destructive testing there can be little confidence in the integrity of a finished field joint coating.